Question

According to Coulombs law if q1 or q2 is zero then the force Fe=0, however if a negatively charged rod is brought close to a neutral metallic knob, electrons on the knob start flowing so there is a force acting eventhough q1 is negative and q2=0. what am I getting wrong here? Thanks

Answer 1

Well a point charge having NO charge is MUCH different than a point charge being neutral.

You are correct that when a negatively charge object is brought to the neutral knob, there WILL be a flow of electrons.

When a negatively charge object touches a neutral object, electrons will leave the negatively charged object ad transfer to the neutral object, thus causing the knob here to have a negative charge

Answer 2

Hi, thanks for the reply. can you elaborate on what the difference is between a point charge having NO charge and point charge being neutral? according to coulombs law shouldnt the force be zero between the rod and the knob? since the chrge of the knob is zero?

Answer 3

this may not be a very accurate way of the actual things but for simplicity here it is:

neutral charge is represented as zero
1 postive and 1 negative = zero
+1 -1 = 0

1 proton and 1 electron = neutral (same as zero)
1p + 1e = neutral charge or zero charge.

but you have to remember that neutrons neither contain a positive nor a negative charge, it is simply neutral, meaning zero charge

while it is good to represent mathematically what goes in the microscopic world, sometimes our language fail in providing an accurate description. you will learn more about this when you take maybe some chemistry lesson and learn more about electrons

Answer 4

I still see a contradiction between coulombs law and the static electrcity experiment. according to coulombs law the force should be zero since the knob is neutral (as in number of electrons = number of protons) however there is a force since the electrons start flowing.

Answer 5

there is no contradiction

Answer 6

http://ocw.mit.edu/high-school/physics/demonstrations-on-video/electrostatics/charge-and-coulombs-law/

Answer 7

Basically the charge it the nob is non-zero.If there is equal no. of +ve and -ve charges then the object is neutral.In metals especially the electrons are not strongly bonded to their nucleus..refer to
http://en.wikipedia.org/wiki/Metallic_bonding
Coming to your question the force to be calculated is between two charged particles considering them individually.
so unless a particle is with zero charge(eg.neutrons,photons) , the force is always applied.
the flow you have mentioned is just because of the consequence of the force you apply is more than the force of nucleus.
analogy to "mass"
1.every body applies force on other body but one could not apply force enough to move it by our gravitational interaction.
2.Earth can show the force as every thing falls towards it.

Answer 8

I take it then, back to Coulombs law, a neutral particle is literally a particle with no charge such as the neutron. It is not referring to a "uncharged" particle such as an atom. In that case there is no contradiction, I get it.
Please confirm if I am right! Thank you all.

Answer 9

@od67 I think you are saying the opposite.....atom is neutral but neutrons are chargeless
1.all chargeless particle can be called neutral( neutron)
2.all neutral systems are not chargeless( eg.atom)

Answer 10

Going back to the first question, when you put a negatively charged rod near a neutral knob, the rod will displace the electrons in the knob so that, the side near the rod becomes positive and the other side becomes negative. This is the source of the flow of electrons. Although the knob is neutral, when they are near, there will be forces. Coulomb's law still works but here, it is localized.

Answer 11

this is dragging on. try opening physics and chemistry books, learn the concepts and solve some problems - these things are taught even at an introductory level.

Answer 12

Od67 - I recommend physicsclassroom.com. The sections on static electricity explain different methods of charging rather well. Without contact the distribution of charges on the door knob will change such that a positive charge is induced at the side nearest to the rod, the resulting polarisation causes greater net attraction between the rod and the knob and may cause electrons to transfer if the electric force is strong enough to drag away electrons from the rod. The door knob becoming polarised means that there is more positive charge closest to the rod by induction but the knob would still be neutral overall. Neutral simply means that there is an equal amount of positive and negative charge over the entire knob. It does not say anything about the charge available within the object, for example the Earth is neutral overall but contains A LOT of charge.

Answer 13

the coulomb's law is only for the interaction between point charges.the interaction between the arrangement of charges are a bit complex.Hence you can't apply coulomb's law equation for a metal rod and the metal knob.But of course you can tell how the charges will behave when a negatively charged rod is brought close to the metal knob.When you bring the negatively charged rod is brought close to the metal knob,the negative charges on the rod repel the free electrons on the knob (as it is a metal there are lots of free electrons).Thus,arranging themselves away from rod.Thus leaving the positive charges at their former positions.These positive charges interact with the negative charges of the rod.Thus attracting each other.

(Excuse me for my bad drawing)